The present invention relates to an image intensifying device, having a photocathode layer arranged on the inner surface of a light-transmissive photocathode carrier and a cathode electrode, which has a first annular electrode part connected electrically to the photocathode layer and a second annular electrode part and which is connected to the negative pole of a direct current voltage source.
Proposals have already been made for protecting electronic image intensifying devices with regard to a sudden sharp increase in the incident light or beam intensity and for protecting the fluorescent screen from being burnt by connecting the photo-emissive layer of the cathode to a good electrically conductive edge and a remaining part of this electrode which acts as a focusing means, via a vacuum deposited resistance layer of such high electrical resistance that, when there is an increase in the intensity, a voltage difference arises between the good conductive edge and one of the focusing means and that the electron beam is thus defocused and/or deflected.
This type of image intensifying device has the disadvantage that, in order to achieve adequate protection (particularly at the center of the fluorescent screen), the properties for transmitting the image are restricted even under normal operating conditions (particularly when observing brightly illuminated objects) by reduced resolution, distorted image reporduction and a restriction in the useful image size. Above all, this is because the image-producing electron beam is already defocused and/or deflected when the photocathode of the tube is acted upon by a beam which is compatible with the anode of the fluorescent screen without any permanent destruction thereof. This is particularly true for the zones of the photocathode adjacent the conductive edge. A further disadvantage is produced if, as usual, the tube is operated by a voltage supply which limits the maxmum current which flows in the tube. With this type of voltage supply the tube cannot be operated even during daylight without damage. A current-limiting voltage supply is used particularly if several image intensifying tubes are placed in a two or three stage cascade arrangement. Current limitation is then selected so that the load of the anode of the fluorescent screen of the image-intensifying tube, which is located at the final stage, is not exceeded.
When using image-intensifying devices of this type, there is the further disadvantage that, when observing bright image scenes, there is a shrinkage of the electron beam from the edge as a result of the potential difference arising between the good conductive edge of the photo-emissive layer and the remaining part of the cathode electrode, acting as a focusing means. With very bright image scenes, the image appearing on the fluorescent screen is reduced to a small fraction of its original dimensions. The current controlled voltage supply limits the current flowing in the tube independent of this focusing effect. As a result of the greatly reduced image size, however, the permissible surface loading of the fluorescent screen is exceeded. In order to avoid this, limiting current in the voltage supply has to be so designed that it limits the current at substantially smaller values. However, this would produce a restriction in the optical output and a narrowing of the range of intensity of scene illumination, in order for the image-intensifying tube to supply useful image information.